In order to correct aberrations in image pickup optical systems, there are used various methods. For example, in an image pickup optical system having a wide angle of field, an aspherical lens is used to perform aberration correction with respect to a beam having a large angle of field. As the image pickup angle of field becomes larger (as the angle of field becomes wider), there occur more frequently coma, field curvature/astigmatism and the like. Thus, in order to correct those aberrations, the number of lenses is increased, and aspherical lenses are used.
Optical elements (lenses) used in an image pickup optical system are unchangeable in shape. For this reason, even if the surface of a lens has a curvature or an aspherical shape optimal for a given angle of field (zoom position) or a focus position, this shape is not necessarily optimal for another angle of field (zoom position) or another focus position.
To address this, there is conventionally known an image pickup apparatus in which aberration correction is performed with an effect similar to the effect obtained when the curvature and the aspherical coefficient are dynamically changed, by interposing a reflective phase modulating element in the optical system and performing optimal wavefront modulation according to each zoom position, focus position or the like (PTL 1). Further, there is known a pickup apparatus for an optical disc in which aberration generated due to a thickness error, a tilt or the like of the optical disc is corrected by using a liquid crystal element (PTL 2).
In general, all aberrations in an optical system can be expressed as a pupil aberration at a pupil of the optical system. Accordingly, there is known an imaging optical system in which a phase modulating element is interposed at the pupil of the optical system to correct the spherical aberration, and conversely add the spherical aberration to obtain a soft-focus effect (PTL 3).
Further, as for the method in which the shape of the reflective phase modulating element is changed in synchronization with the zoom position or the focus position to correct aberration in an image pickup optical system, as the angle of field becomes wider, the aberration correction for the entire screen becomes more difficult with only a single aspherical shape. For this reason, this method tends to leave many residual aberrations.